The present invention relates to a piezoelectric transformer; and, more particularly, to a step-down type multilayer piezoelectric transformer.
Piezoelectric transformers feature smaller sizes and higher efficiencies than the conventional electromagnetic transformers. For such reasons, the piezoelectric transformers are gaining popularity as step-up transformers employed in, e.g., backlight power sources of liquid crystal display (LCD) monitors. However, in power supplies, step-down piezoelectric transformers are more frequently used than step-up transformers.
Voltage transformation by the piezoelectric transformer is performed as follows. First, an electrical input signal is applied to an input part of the piezoelectric transformer. Next, the electrical input signal is transformed into the mechanical vibration at the input part of the piezoelectric transformer and the mechanical vibration is propagated to an output part thereof. Then, the mechanical vibration is changed into an electrical output signal at the output part of the piezoelectric transformer.
It is preferable that a piezoelectric transformer have such characteristics as high output power, less heat generation, small size and high efficiency. The output power of a piezoelectric transformer is dictated by the velocity of the mechanical vibration. In order to generate a high vibration velocity, the input part of the piezoelectric transformer need be driven to effect a large strain. This can be accomplished by increasing interface areas between the piezoelectric material and the electrodes, e.g., by arranging the electrodes and the piezoelectric material to form a laminated structure or increasing the areas of the electrodes.
However, when the piezoelectric transformer is driven to effect a large strain, heat generation is increased due to crystallographic and electrical losses and the output power is decreased by the generated heat. Most of the heat is generated from the input part of the piezoelectric transformer. Therefore, when the input part is under restricted conditions due to heat generation, the transformation efficiency of the output part has relatively greater effects on the electrical characteristics, e.g., the output power, of the piezoelectric transformer. For such reasons, in order to produce a high output power without increasing heat generation, the mechanical vibration generated in the input part need be efficiently transformed into electrical signals at the output part.
The output power P of the piezoelectric transformer is represented as follows:   P  ∝                    k        2                    1        -                  k          2                      ·    V    ·          v      2      
where k is an electromechanical coupling coefficient of the output part of the piezoelectric transformer, v is the velocity of the mechanical vibration and V is the volume of the output part. Therefore, the output power of the piezoelectric transformer can be enhanced by increasing the volume of the output part thereof. However, there is an upper limit for the volume of the output part which can be activated by the given amount of the vibration energy supplied from the input part. Further, the size of the piezoelectric transformer becomes larger by increasing the volume of the output part, which is not desirable.
It is, therefore, a primary object of the present invention to provide a piezoelectric transformer having a laminated structure and capable of producing a higher output power without increasing heat generation and the device size thereof.
In accordance with a preferred embodiment of the present invention, there is provided a piezoelectric transformer having a laminated structure, comprising one or more input parts including first internal electrodes and input terminals, and an output part including second internal electrodes and output terminals, wherein each of the second internal electrodes has an area larger than that of each of the first internal electrodes.
In accordance with another preferred embodiment of the present invention, there is provided a piezoelectric transformer having a laminated structure, comprising a laminated body including one or more input parts and an output part, the input parts being formed by laminating a plurality of first piezoelectric sheets and having first internal electrodes each of which is formed on each of the first piezoelectric sheets and input terminals for the first internal electrodes which are alternately exposed to opposite sides of the laminated body, the output part being formed by laminating a plurality of second piezoelectric sheets and having second internal electrodes each of which is formed on each of the second piezoelectric sheets and output terminals for the second internal electrodes which are alternately exposed to another opposite sides of the laminated body, and external electrodes which are electrically connected to the input terminals and the output terminals.